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Static metal-semiconductor contacts are classified into Ohmic contacts and Schottky contacts. As for dynamic metal-semiconductor contacts, the in-depth mechanism remains to be studied. We here define a "triboelectric junction" model for analyzing dynamic metal-semiconductor contacts, where a space charge region induced by the triboelectric effect dominates the electron-hole separation process. Through theoretical analysis and experiments, we conclude that the triboelectric junction affects the electric output in two aspects: 1) the junction direction determines the output polarity; 2) the junction strength determines the output amplitude. The junction direction and junction strength are both related to the electron-affinity difference between the contact metal and semiconductor. We find that the standard electrode potential in electrochemistry best describes the electron affinity of a dynamic metal-semiconductor contact.